设 $\theta$ 是第二象限的角, 则必有
A
$\operatorname{tg} \frac{\theta}{2}>\operatorname{ctg} \frac{\theta}{2}$
B
$\operatorname{tg} \frac{\theta}{2} < \operatorname{ctg} \frac{\theta}{2}$
C
$\sin \frac{\theta}{2}>\cos \frac{\theta}{2}$
D
$\sin \frac{\theta}{2} < \cos \frac{\theta}{2}$
E
F